Lightning strikes are a major reason for wind turbine failures. When a lightning strikes a wind turbine often one or more blades of the wind turbine are hit. From there the current of the lightning strike is usually conducted by conducting means like a down-conductor to one or more wind turbine components like the hub, nacelle or the tower and then further down to the ground.
The current flowing through the wind turbine must not reach any electrical components like the generator or converter or other critical components like the main bearings or the gearbox as they would be damaged by the large energy of the lightning strike. The combination of a high voltage and a large electrical current from the lightning strike could damage these components.
Often, the down-conductor is connected directly to the hub which conducts the current to the nacelle and further down to the ground. For a direct drive wind turbine also called gearless wind turbine the hub is connected electrically and mechanically to the rotor of the generator. Due to this design the current can pass onto the rotor and discharge into the stator of the generator. The large current of up to 200 kA could destroy the permanent magnets, the generator windings and could introduce metallic particles between rotor and stator. The particles will stick to the magnets and decrease the distance between rotor and stator which might cause contact between both generator parts. Therefore, lightning protection systems like these are not applicable for direct drive wind turbines.
For a direct drive wind turbine having a generator with an outer rotating rotor it is even more difficult to find a way to conduct the electrical current from a lightning strike to the blades without damaging the generator or other components of the wind turbine.
U.S. Pat. No. 7,377,750 B1 shows a lightning protection system for conducting lightning strikes to the blades and further to the region surrounding a blade hub and along a path around the blade hub and critical components of the wind turbine. The long lightning conducting path has directional shifts or turns bearing the risk of lightning strike jumps to a critical component.